1. Field of the Invention
This invention relates to artificial joints of the ball and socket type and in particular to fixation elements for use with such joints.
2. Description of the Prior Art
As is well known in the art, artificial hip and shoulder joints conventionally employ ball and socket articulations. The socket portion of the joint is embedded in one bony structure, for example, the pelvis for a hip reconstruction. The ball portion of the joint is attached to another bony structure, for example, the femur for a hip reconstruction.
Over the years, a variety of techniques have been used for embedding the socket portions of these joints in bony structures. Broadly, these techniques fall into two classes: fixations which use cement and fixations which do not use cement. In general, cementless fixations, and especially cementless fixations where metal interfaces directly with bone, have the potential for producing a longer lasting and stronger prosthesis fixation because they do not include cement which can deteriorate with time and which is relatively weak in comparison to metal and bone.
Cementless reconstructions of the hip socket are themselves divided into two classes. In the first class, attachment is achieved by impacting the socket portion of the joint into a prepared cavity in the bony structure. U.S. Pat. No. 3,820,167, issued to K. M. Sivash on June 28, 1974, shows one such impaction construction. In the other class, the socket portion of the joint includes a screw thread on some portion of its outer surface which is used to screw the prosthesis into a prepared cavity in the bony structure. One such screw-in socket made out of ceramic material is the cement free "Autophor" system sold by Osteo AG (Selzach, Switzerland). The present invention relates to such screw-in type sockets.
The screw-in sockets which have been used to date require a three step process for implantation. First, a cavity having the shape of a truncated cone and a cone angle and diameter appropriate to the cone angle and diameter of the threads on the outer surface of the prosthesis and a depth corresponding to the height of the prosthesis is reamed into the bony structure. Next, a thread having a pitch and thread type corresponding to the pitch and thread type on the outer surface of the prosthesis is tapped into the wall of the cavity. Finally, the prosthesis is screwed into the cavity, the threads on the outside of the prosthesis mating with the threads previously tapped into the wall of the cavity.
This implantation procedure has numerous disadvantages, which, as described below, the present invention overcomes. First, and most importantly, the procedure results in the removal of a relatively large amount of bone from the bony structure because the bone's natural shape at the place where the reaming is done is generally spherical, not conical. For example, for hip reconstructions, the cotyloid cavity (also known as the acetabulum), even for severely deformed joints, is generally spherical in shape. To ream a truncated cone out of a sphere requires the removal of a substantial amount of bone, which bone, once removed, obviously cannot be replaced.
Along the same lines, the process of reaming a truncated cone into a spherical cavity in a bony structure runs the risk of puncturing through the bony structure, especially in the areas where large amounts of bone must be removed to change the spherical contour into the desired conical contour. For example, as shown in FIG. 1, the reaming of a conical cavity into cotyloid cavity 1 can result in penetration through to the internal surface of one or more of the ilium 2, ischium 3 or os pubis 4 bones. In particular, in the region of the fossa acetabuli at the bottom of the cotyloid cavity, the ischium and os pubis bones are especially susceptible to penetration during conical reaming because these bones are thinnest in this region.
In addition to the problems caused by bone removal, the currently used procedure also suffers from the disadvantage that the conical reaming must be performed precisely both to insure a tight fit between the prosthesis and the wall of the cavity and because the axis used for reaming defines the final orientation of the socket portion of the prosthesis with respect to the ball portion. Correction of misdirected reaming has generally involved re-reaming the cavity using a larger diameter reamer and then either using a larger socket or employing cement to retain the original socket in the now oversized cavity. Re-reaming is obviously undesirable since it involves the removal of more bone and an enhanced risk of penetration of the bony structure, as well as increasing the time the patient spends under anesthesia.
The prior screw-type prostheses also suffered from the disadvantage of limited options if a secure fixation was not achieved when the socket was screwed into the prepared cavity in the bony structure. Such an insecure fixation could result from the bony structure being composed of especially soft or porous bone. In such a case, the required level of fixation was generally achieved by the use of cement or, in some cases, by re-reaming and retapping the cavity at a larger diameter in the hope of reaching sufficiently strong bone to provide the required level of fixation.
In addition to the foregoing, the use of three separate steps--reaming, tapping and screwing-in--made implantation of prior screw-in type sockets a relatively lengthy procedure in comparison with, for example, the implantation of impaction type prostheses.